This invention relates to Polymer Electrolyte Membrane (PEM) fuel cells and, in particular, to modular PEM fuel cells designed for assembly into fuel cell stacks. Additionally, the invention relates to fuel stack apparatus and methods with improved maintenance, scalability, and heat dissipation characteristics.
Generally, fuel cells for the production of electrical energy from a fuel and oxidant are known in the art. In a fuel cell, electric power and water vapor (as a by-product) are produced when fluid hydrogen and oxygen, usually in the form of gases, provided to anode and cathode electrodes respectively, react through an electrolyte. Electric power produced is then collected by the lead lines. Essentially, the reaction is an oxidation of the fuel, but the method results in direct production of electrical energy, with heat energy being produced as a side effect. As an alternative to hydrogen gas, other fuels containing hydrogen may be used.
A good deal of concern in the art is directed to methods of stacking unit fuel cells in order to increase overall voltage output. A single cell, depending on design and application, may provide 0.3-1.5 volts output. Most battery replacement applications, for example, require 1.5-24 volts or more output voltage. The favored approach to increase voltage from that provided by a single cell is to physically stack a plurality of unit cell structures. This is commonly done with a sequentially stacked first gas diffusion layer, an anode electrode film, an electrolyte layer, a cathode electrode film, a second gas diffusion layer, and a bipolar connecting plate. The unit cells are electrically connected in series by means of conductive end plates. Generally, one end plate contacts the anode electrode of a first unit cell on one side of the stack, and another end plate contacts the cathode electrode of a last unit cell on the opposite side of the stack, with any number of similar cells stacked between.
In operation, hydrogen gas or other fuel is provided in the anode side of the fuel cell body, oxygen gas as oxidant is provided in the cathode side. The hydrogen and oxygen then react, producing a useful electric current, and water vapor as a by-product. The electrolyte can be a solid, a molten paste, a freeflowing liquid, or a liquid trapped in a matrix. The solid type of electrolyte, or Polymer Electrolyte Membrane (PEM), is well known in the art.
In the construction of fuel cells, several tradeoffs are made in order to optimize overall function in light of problems with the existing state of the art. Most currently known PEM fuel cells use a construction technique which incorporates the functions of anode electrode film, electrolyte, and cathode electrode film into a single unit, called a Membrane Electrode Assembly (MEA). This has the advantage of making assembly of stacked fuel cells more convenient. The conventional Gas Diffusion Assembly (GDA) provides for routing of gases to the MEA, and also for electrical contact with the MEA.
In the conventional PEM fuel cell stack of conventional unit fuel cells, a compressive force is applied to the end plates. This is transferred as a distributed force to the individual members of the stack. There may be many individual members of the stack, depending on the desired voltage output. For instance, if a single unit cell provided 0.6 volts, and a total of 24 volts output were desired, then there would be 40 unit cells in the total stack. A number of problems can occur as a consequence of this reliance on a distributed applied force to squeeze the stack together.
One such problem is that during operation, the membrane included in each MEA changes volume depending on both operating temperature and degree of hydration. The degree of hydration varies depending on operating circumstances. During the chemical reaction in the fuel cell, water vapor is produced as a by-product. This water vapor may back-diffuse from the cathode through the electrolyte, resulting in a substantial volume change in the electrolyte. In fact, the water cannot be completely eliminated, but rather, its presence in the electrolyte layer is helpful in maintaining high protonic conductivity. Therefore, the fuel cell design must accommodate substantial volume change of the electrolyte layer.
There exist tradeoffs between the need to accommodate swelling of the electrolyte layer, and the need for good electrical contact and fluid sealing. In the fuel cell stack design, a certain amount of compression is provided in order to ensure good electrical contact between adjacent unit fuel cell structures, as well as to maintain sealing of the fuel and oxidant fluids. As the volume change occurs, the effective xe2x80x9csqueezexe2x80x9d or compressive pressure applied will vary. For a large number of stacked unit cells, the variation in distributed squeeze pressure can be pronounced. When squeeze pressure varies on the low side, then either leakage of supply fluids can occur, or electrical contacting resistance can increase. Both result in a loss of energy efficiency. When squeeze pressure varies on the high side, then catastrophic damage to one or more unit cells can result. Repair involves complete disassembly of the stack, troubleshooting to locate the damaged unit cells, replacement of damaged unit cells, and re-assembly of the stack. Such repairs can be quite costly, and add significantly to the life-cycle costs of the fuel cell stack.
Conventional approaches to fuel cell stack construction include systems for dissipating excess heat. A significant portion of the available energy output of a fuel cell stack can in fact be diverted to cooling fans or compressors in order to handle the heat load. In fact, fuel cell stack applications can be limited by the requirement to dissipate waste heat. Either power output must be reduced in order to prevent uncontrolled heat buildup, or a cooling system must be provided. A cooling system carries the serious disadvantages of cost, size, complexity, and loss of overall system thermodynamic efficiency.
There is a need for fuel cells designed for creating fuel cell stacks which overcome these problems, particularly in portable applications.
A Polymer Electrolyte Membrane (PEM) fuel cell assembly apparatus has a membrane electrode assembly (MEA). The apparatus has a body with a cavity for receiving the MEA and conductive elastomeric seals. The seals divide and hermetically separate the body cavity into a cathode chamber and an anode chamber. A fuel port is provided in the body for conducting fuel into the anode chamber. The cathode chamber is exposed to a source of oxygen. An integral connector included in the body provides an electrical path between the anode chamber and the outside of the body. An anode conductor provides an electrical path between the anode surface and the integral connector. A cathode conductor provides an electrical path between the cathode surface and an external connector, which extends outside of the body. In the preferred embodiment, a lid is provided for completing assembly and for maintaining compression of the components to maintain hermetic sealing and good electrical connections. The apparatus includes modular features for connecting a plurality of fuel cells into a fuel cell stack, and for facilitating removal and replacement of individual cells or MEAs in a stack. The fuel cell and stack are designed for inherent heat dissipation characteristics.
According to another aspect of the invention, individual fuel cell assemblies are incorporated into one integrated stack unit having multiple cavities. The integrated stack units are themselves stackable.
According to yet another aspect of the invention, a fuel cell stack with individual fuel cells has lids incorporated in one integrated unit having multiple apertures.
According to still another aspect of the invention, a fuel cell has snap connectors for mechanically and electrically coupling the fuel cell to one or more additional fuel cell to form a modular fuel cell stack.
The present inventions offer definite advantages over conventional systems and methods, particularly in terms of scalability, modularity, and the ease of configuring fuel cell stacks for particular voltage, size, shape, and heat dissipation characteristics according to the desired application.